The purpose of the network interface filter is to allow for filtering
of messages (accept or reject) receive and send. This is done
on two levels:

The first level is at the UDP entry / exit point, i.e.
immediately after the receipt of the message, before any message
processing is done (accept_recv) and
immediately before sending the message, after all message
processing is done (accept_send).

The second level is at the MPD entry / exit point, i.e.
immediately after the basic message processing (accept_recv_pdu) /
immediately before the basic message processing (accept_send_pdu).

Note that the network interface filter is something which is used
by the network interface implementation provided by the application
(snmpm_net_if and snmpm_net_if_mt).
The default filter accepts all messages.

A network interface filter can e.g. be used during testing or for load
regulation.